Bilayers of yttrium and amorphous silicon have been irradiated with 60 keV inert ions. Between liquidhyphen;nitrogen temperature and 100thinsp;deg;C, ion mixing resulted in an amorphous alloy of Y and Si. For temperatures of 125ndash;190thinsp;deg;C, we observed formation of the YSi phase. YSi is not formed during thermal anneals of bilayers. Ion mixing at higher temperatures (ge;205thinsp;deg;C) results in the formation of the stable YSi1.7phase. Such sequential silicide formation has not been observed for comparable rarehyphen;earth silicides. The minimum temperatures for ionhyphen;induced YSi1.7formation agrees with the prediction by a simple model which correlates vacancy mobility to phase transformation. The YSi formation temperature is associated with the onset of radiationhyphen;enhanced diffusion. This temperature does not correlate well with the prediction of the model, but agrees with a scaling based on the average cohesive energy.
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